The gimppaintcore-loops functions perform very little actual
computational work (in case of do_layer_blend(), at least for
simple blend modes), which makes the cost of buffer iteration, and
memory bandwidth, nonnegligible factors.  Since these functions are
usually called in succession, acessing the same region of the same
buffers, using the same foramts, coalescing them into a single
function, which performs all the necessary processing in a single
step, can improve performance when these functions are the
bottleneck.

Add a gimp_paint_core_loops_process() function, which does just
that: it takes a set of algorithms to run, and a set of parameters,
and performs all of them in one go.  The individual functions are
kept for convenience, but are merely wrappers around
gimp_paint_core_loops_process().

Be warned: the implementation uses unholy C++ from outer space, in
order to make this (sort of) managable.  See the comments for more
details.